Immobilin, a novel glycoprotein (mucin) of high molecular weight found in rat cauda epididymal fluid, inhibits the initiation of motility of rat sperm. Immobilin appears to inhibit sperm motility by virtue of its viscoelasticity rather than by its biochemical properties. A number of physiologically and biochemically important questions about immobilin remain unanswered: 1. Relationship of immobilin's structure to its inhibition of sperm motility. The possible correlation between immobilin, its viscoelasticity and its ability to inhibit sperm motility will be examined. Immobilin will be purified to homogeneity and its amino acid and carbohydrate composition determined. Immobilin will be chemically and enzymatically modified, and the effects of each modification on immobilin's biological activity will be determined. 2. Biogenesis of immobilin. Polyclonal antibodies to immobilin will be generated in rabbits and used as probes: a) Site of synthesis of immobilin. The tissue and cell specificity of immobilin synthesis will be examined both biochemically, by immunoblotting and immunoprecipitation of homogenized tissues, and morphologically, by immunolabeling tissue sections. b) Regulation of immobilin biosynthesis. Once the tissue and cell specificity of immobilin biosynthesis have been determined, the ontogeny of immobilin biosynthesis will be examined. Homogenates of the appropriate organ from fetal, neonatal, pubertal and adult rats will be immunologically screened for immobilin. In addition, whether the initial appearance of immobilin protein in the lumen of the cauda epididymis is due to the concomitant appearance of functional (translatable) immobilin mRNA will be indirectly determined by in vitro translations and immunoprecipitation of newly synthesized immobilin. The possible hormonal regulation of immobilin biosynthesis will be investigated in vivo. The hormonal status will be manipulated by hypophysectomy, castration, efferent duct ligation, and androgen replacement therapy. Whether androgens exert their effect by increasing the levels of functional immobilin mRNA will be examined by in vitro translation and immunoprecipitation of newly synthesized immobilin.